Corrugated board is typically prepared by a process known as the Stein-Hall process. As is generally described in U.S. Pat. No. 2,102,937 (issued to J. V. Bauer), the Stein-Hall process employs a corrugating adhesive to bond a corrugated paper "medium," such as a roll or strip, to a liner board on one or both sides of the corrugated medium. Adhesives that are used in conjunction with the Stein-Hall process long have been known, and such adhesives generally comprise an aqueous emulsion of raw starch; caustic; pasted modified or unmodified starch; and a cross-linking agent. The raw starch serves as a binder in the finished corrugated board, while the pasted starch and cross-linking agent form a tacky composition that holds the plural layers of green corrugated board together before the primary starch adhesive has set. The cross-linking of the pasted starch is further thought to impart suspending power and to affect the viscosity of the corrugating adhesive during storage and application.
Known corrugating adhesives suffer from a number of drawbacks. For example, the speed of the machinery used to prepare the corrugated board sometimes is limited by the rheological properties of the adhesive. During manufacture of corrugated board, the corrugating adhesive typically is spread across the liner board or the corrugated paper medium with a spreader knife or metering roller. It has been observed that conventional adhesives undergo substantial shear thinning when they are spread too quickly, thus leading to problems in applying the adhesive to a corrugating medium in conventional corrugating equipment. The shear thinning thus may serve to limit the speed of the corrugating equipment, and thus may limit the attainable output of corrugated board.
Another drawback relates to the green bonding strength of conventional corrugating adhesives and, more specifically, to the rate at which the tack of the adhesive increases when the corrugated board is in the green state. Typically, corrugated board is processed and handled before the adhesive has fully dried, the adhesive thus being in the green state. If the adhesive has not become tacky quickly enough, then the corrugated board will delaminate during the processing operations that follow the bonding operation. The rate of increase of tackiness of known adhesives thus may be a further limiting factor in the rate of manufacture of corrugated board.
It is believed that the Theological instability of conventional corrugating adhesives can be attributed to the exclusive reliance of the adhesive composition on pasted starch to provide tack in the green state. Recent prior art has taught to replace a portion or all of the pasted starch in the corrugating adhesive with hemicellulose, a plant derivative obtained from a hemicellulose-containing plant source such as corn hulls. For example, U.S. Pat. Nos. 5,358,559 and 5,503,668 each purport to provide a corrugating adhesive composed of starch and hemicellulose, the hemicellulose being derived from corn hulls. Similarly, U.S. Pat. No. 5,777,005 purportedly provides a corrugated starch based on a combination of corn hulls and tapioca fiber. Each of these patents teaches to blend corn hulls with starch and to extract hemicellulose in-situ from the corn hulls to provide a hemicellulose-based adhesive composition.
While such recent art purports to provide hemicellulose-containing corrugating adhesives, these adhesives are unsatisfactory in some respects. For example, the rate of increase of tack of these adhesives when in green state is unsatisfactory, thus potentially leading to delamination problems if the corrugating operation is run at too high a speed. Thus, while the rheological stability of such corrugating adhesives may be improved as compared with those of corrugating adhesives based on pasted starches, these adhesives still are of limited value in increasing the rate of production of corrugated board.
An improved hemicellulose-based adhesive, and relative advantages thereof as compared with known adhesives, are described in copending U.S. patent application Ser. No. 09/182,918, filed Oct. 30, 1998, and assigned to Grain Processing Corporation of Muscatine, Iowa. As disclosed therein, hemicellulose is obtained from the soluble phase of an extract obtained from hydrolysis of a hemicellulose-containing plant source. The hemicellulose-containing soluble phase is separated from insoluble portions of the extract and is blended with starch to form an adhesive composition. The improved hemicellulose-based adhesive described in U.S. Pat. No. 6,063,178 has significant advantages over known hemicellulose-based adhesives, such as improved green tack properties. Despite these advantages, however, even these soluble phase hemicellulose-based adhesives exhibit somewhat higher water-retention properties than is sometimes desired (albeit to a lesser degree than known hemicellulose-based adhesives). High water retention slows the rate of increase of tackiness when the corrugated board is in the green state, and thereby limits the rate of manufacture of the corrugated board.
In view of the foregoing, there exists a need in the art for a hemicellulose-based corrugating adhesive that has improved green tack property and reduced water-retention relative to conventional corrugating adhesives. The present invention provides such an adhesive. These and other advantages will be apparent from description of the inventions provided herein.